1. Field of the Invention
The present invention relates to a remaining battery capacity display device for use in a communication device powered by a battery, and more particularly, to a remaining battery capacity display device suitable for a mobile terminal employed in a CDMA (Code Division Multiple Access)-based mobile communications network.
2. Description of the Related Art
In general, it is a matter of great concern to an operator of an apparatus powered by a battery to be informed of remaining time available for operation of the apparatus. The present invention is concerned with display of such remaining time or remaining capacity of the battery.
Conventionally, the voltage value of the battery is used in estimating remaining operating time available for operation of apparatuses of the above-mentioned type. FIG. 11 shows an example of curves representative of current-voltage characteristics of a lithium battery exhibited at respective levels of the remaining capacity of the battery. As shown in FIG. 11, if the current consumption of the battery is constant, one-to-one correspondence holds between the voltage value of the battery and the remaining battery capacity. Therefore, it is possible to measure the battery voltage and then calculate the remaining battery capacity based on the measured value of the battery voltage. Further, since the current consumption is constant, it is possible to estimate remaining operating time of the device.
However, in the case of a mobile terminal powered by a battery, the passage of battery current through a transmitter, which consumes electric current more than any other component thereof, is permitted only during signal transmission, but is inhibited during standby for signal reception so as to prevent waste of remaining capacity of the battery. As a result, the battery current consumption during standby for signal reception is different from that during transmission. For that reason, in the mobile terminal, it is impossible to estimate remaining battery capacity or remaining operating time simply from a measured value of the battery voltage. Therefore, the mobile terminal employs two different methods of estimating remaining battery capacity, i.e. one used during standby for signal reception and the other used during signal transmission. For instance, the remaining battery capacity or remaining operating time is estimated such that the estimated value is smaller during signal transmission than during standby for signal reception.
In a CDMA-based mobile communications network, it is essentially required that signals transmitted from respective mobile terminals are received at the lowest possible level at each base station thereof. To this end, the base station always sends each mobile terminal a signal based on which the mobile terminal should adjust the level of a signal transmitted therefrom. Alternatively, a mobile terminal measures a reception level of a signal received from the base station, estimates an appropriate transmission level from the measured reception level, and then sends a signal to the base station at the estimated appropriate level. Therefore, the signal transmission level of the CDMA-based mobile terminal frequently changes as the terminal moves to change its position.
For the above reason, the battery current consumption by the CDMA-based mobile terminal is not constant even during transmission. Moreover, the transmission level of a signal from the terminal is required to be adjustable e.g. within a range of 20 dB to 70 dB. Therefore, the remaining battery capacity display device used in the conventional mobile terminal described above, which operates on the precondition that the battery current consumption is held constant during transmission, is not capable of displaying an exact remaining battery capacity for the CMDA-based mobile terminal.